FWIW, I saw on Whittaker's FaceBook page today they had a NIB Sig P226 MK-25 for $799.99. I'm not sure what the differences are between that one and the one you're talking about but figure $800 + FFL fee + shipping (I'm not sure what Whittaker's charges) and now you have something to compare to the one you're looking at--assuming they're reasonably close.
Here's the specs for the MK25 for comparison purposes:
P226 MK25
If you could pick up the MK25 for 800 NIB that is a great deal. I paid 999 for mine.